1. Field of the Invention
The present invention relates to a computerized visual contrast sensitivity function measurement system, and more particularly to a computerized video system configured to implement a tilted-grating, forced choice contrast sensitivity function test. Even more particularly, the present invention utilizes known measurement methods for the visual contrast sensitivity function and automates their use by computerizing the system and couples it with a patient-interactive user interface that records the shape of a results curve producing an accurate quantitative result.
2. Discussion of the Related Art
Simply speaking, contrast is a measure of the amount of lightness or darkness an object has relative to its background, or in other words, it is the difference in luminance that makes an object distinguishable. For example, a black letter on a white background has more contrast than a black letter on a gray background. The contrast threshold is the smallest difference of the lightness and darkness between an object being viewed and its background that can be distinguished by a viewer. Contrast sensitivity is the inverse of the contrast threshold which defines the threshold between the visible and the invisible, and thus any reduction in contrast sensitivity may impair daily activities, including reading, utilizing tools, driving, and simply finding objects. There are diseases that may cause a loss of contrast sensitivity as well as improperly designed optics, including spectacles and contact lenses.
An object's size, or more specifically, the angle it subtends in space, influences how much contrast is required to differentiate an object from its background. The size of an object may be depicted by alternating lines of light and dark with an appropriate spacing occupying a specific visual angle. The number of light and dark lines within the specific visual angle is referred to as the spatial frequency. Densely packed lines represent a high spatial frequency while sparsely packed lines represent a low spatial frequency. The relationship between contrast sensitivity and spatial frequency is known as the contrast sensitivity function. Determining an individual's contrast sensitivity function may provide a valuable clinical adjunct to standard visual acuity tests or measurements. More specifically, whereas acuity is a test of visual resolution or detection and the maximum frequency detectable, the contrast sensitivity function tests performance across a range of spectral frequencies and this range can be correlated to subjective visual satisfaction.
It is generally accepted that contrast sensitivity function (CSF) measurements are an important and informative method for assessment of the human visual system. However, these measurements are only infrequently made in research environments and rarely in the clinical setting. The primary reason it is rarely utilized is the length of time required to complete an effective measurement. It is simply impractical as a routine test. In addition, the length of time required for the measurement may lead to measurement inaccuracies due to subject fatigue.
The traditional CSF test employs a series of images or test charts with various spatial frequencies and contrast levels. Typically, a large number of these individual charts are shown to the subject who must make a forced choice as to the presence of a pattern. From these choices, and the long process, the CSF can be determined; however, subject fatigue is often a factor in accuracy as set forth above.
The basic components of a two alternative forced choice test or task includes two alternative choices, for example, two possible visual stimuli, a delay interval to allow a response/choice, and a response indicating a choice of one of the two possible stimuli. For contrast sensitivity function testing, the preferred stimuli are a series of targets consisting of sinusoidal gratings of differing spatial frequency and contrast. To provide a detection method, the gratings are tilted either slightly to the left or to the right or are vertical. For each target the subject must respond with the orientation, even if it is just a guess. The CSF is determined when the threshold between “seeing” and “not-seeing” is determined to some level of precision for each spatial frequency of interest.
The vision Contrast Test System (Vistech) and the Functional Acuity Test System (Vision Science Research Corporation) are commercially available charts produced to implement this test. Examples of these charts are shown in FIGS. 1A and 1B respectively. However, only a small number of charts are used and while the subject must respond for each patch, the patches are presented in order and not as a single test. The presence of other patches as well as spatial variations, such as in illumination, can influence the result.
A variety of other chart-based tests have been demonstrated and a good review can be found by Richmman, Spaeth and Wirostko (“Contrast sensitivity basics and a critique of currently available tests,” J Cataract Refract Surg 2013; 39:1100-1106). The tests described generally present the subject with two or more patches and the subject must determine which contains the pattern. Since a fixed series of patterns, such as illustrated in FIGS. 1A and 1B are used, the test is subject to the influences of learning. Some of these tests have also been implemented with a video apparatus.
Accordingly, there exists a need for a patient-interactive visual contrast sensitivity function measurement system and method that can rapidly assess visual performance, increase the accuracy of the test and produce a quantitative result that may be utilized to design better optics for patients.